If you want a physical satisfaction then keep doing what you have been doing with your nephew without thinking that you are doing something wrong. If certain things are wrong in the eyes of society then it does not mean that they are wrong in your situation too. Sometimes we have to take a decision according to the situation we are in and your situation justifies your act so I would not think what you are doing is morally or socially wrong.
However, you have to be extra careful to keep it secret from everyone otherwise; you will be in a big trouble. If your nephew has crossed the legal age of having sex then you can have sex with him if it is mutual. You do not have a right to compel anyone but if he is willingly wants to share a bed with you then you should do it considering him adult enough to understand what he has been doing.
I would say you are favoring your nephew by giving him your pussy as most of boys want to have sex with mature women and few are lucky enough to get their dream, fulfilled. Do not analyzing too much whether it is right or wrong because you would never able to find the right answer but if you are happy being fucked by him then keep doing it keeping all the moral ethics aside.